Novel concept for exciting and probing coherent phonons in crystals of a transiently broken symmetry

Crystals consist of atoms arranged in a regular lattice, allowing them to move slightly from their equilibrium positions. These atomic vibrations, known as phonons, can be described as quantum states. By combining multiple phonon states, a phonon wavepacket is formed, which corresponds to the collective, coordinated oscillations of the atoms in the crystal.

Coherent phonons can be generated by exciting a crystal with an extremely short burst of light called a femtosecond light pulse. The subsequent motion of the coherent phonons in both space and time can be observed by scattering an ultrashort X-ray pulse off the excited material. The resulting pattern of scattered X-rays directly reveals the instantaneous positions of the atoms and the distances between them. By capturing a series of these patterns, a “movie” of the atomic motions can be obtained.

The characteristics of coherent phonons are determined by the crystal’s symmetry, which arises from its periodic arrangement of identical unit cells. When a crystal is weakly optically excited, its symmetry properties remain unchanged. As a result, coherent phonons with identical atomic motions in all unit cells are excited. In contrast, strong optical excitation can disrupt the crystal’s symmetry, causing atoms in neighboring unit cells to oscillate differently.

Although this phenomenon has the potential to access additional phonons, it has not been extensively explored until now.

A recent study published in the journal Physical Review B presents a groundbreaking approach developed by researchers from the Max-Born-Institute in Berlin and the University of Duisburg-Essen. They have demonstrated a novel method for exciting and investigating coherent phonons in crystals with temporarily broken symmetry. The key to this concept lies in deliberately reducing the crystal’s symmetry through appropriate optical excitation. The researchers successfully applied this technique to bismuth (Bi), a prototypical crystalline semimetal.

(a) Coherent phonon oscillations with a frequency of 2.6 THz observed in optical pump/femtosecond x-ray diffraction probe experiments for different pump fluences of the mid-infrared excitation pulses centered at a wavelength of 5 µm. The phonon wave packets are exclusively observed for strong excitation pulses, i.e., they are absent for pump fluences below 1.9 mJ/cm2. Thus, the reduction of the symmetry of the unit cell via strong optical pumping is necessary to get access to the phonon motion. (b) Spectrum of the phonon oscillation gained by a Fourier transform of the transient at a fluence of 2.9 mJ/cm2 shown in panel (a). Credit: MBI/M. Runge

When ultrafast mid-infrared excitation is applied to bismuth (Bi), it leads to a temporary modification of the spatial charge distribution, resulting in a reduction of the crystal’s symmetry. This reduction in symmetry creates new quantum pathways for exciting coherent phonons. Interestingly, the change in symmetry causes the unit-cell size to double, transitioning from a red framework with two Bi atoms to a blue framework with four Bi atoms. With this doubled unit cell, coherent phonon wave packets with bidirectional atomic motions become possible, in addition to the usual unidirectional motion.

By employing femtosecond X-ray diffraction to directly probe the transient crystal structure, researchers observe oscillations in the diffracted intensity that persist for picoseconds. These oscillations stem from the coherent motion of wave packets along phonon coordinates within the crystal of reduced symmetry. Notably, the frequency of these oscillations (2.6 THz) differs from that of phonon oscillations occurring at low excitation levels. This behavior emerges only beyond a threshold of optical pump fluence and showcases the highly nonlinear and non-perturbative nature of the optical excitation process.

To summarize, the ability to break the crystal’s symmetry through optical means enables the modification of a crystal’s excitation spectrum on ultrafast timescales. These findings hold promise for dynamically manipulating material properties and implementing new functionalities in the realms of optoacoustics and optical switching.

Source: Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI)

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